Tetrafluorosilane (hereinafter sometimes referred to as “SiF4”) is used, for example, as a raw material for optical fibers, semiconductors or solar cells, and a high-purity product is required. As for the production process thereof, for example, a method of producing SiF4 by reacting SiO2 and HF in the presence of concentrated sulfuric acid is known (Japanese Unexamined Patent Publication No. 57-135711 (JP-A-57-135711)).
However, this method has a problem in that water is produced as a by-product upon reaction of the raw materials SiO2 and HF. The water produced may be removed by a concentrated sulfuric acid but cannot be completely removed and the SiF4 produced disadvantageously contains a large amount of HF and hexafluorodisiloxane ((SiF3)2O) produced by the reaction of water and SiF4 and, further, contains carbon dioxide which is difficult to separate from SiF4 and which is considered to originate from the slight amount of a carbon compound contained in the concentrated sulfuric acid.
Also, a method for producing SiF4 by thermally decomposing a hexafluorosilicate is known. However, the hexafluorosilicate contains H2O or impurities such as trace oxygen-containing silicic acid compounds (e.g., SiO2) and unless pretreated satisfactorily, the impurity may react with SiF4 to produce hexafluorodisiloxane when the thermal decomposition is performed.
A method for purifying SiF4 containing (SiF3)2O , CO2 or HF is also known. In the case where SiF4 contains impurity gases such as (SiF3)2O, CO2 and O2, it is known that if the SiF4 is used, for example, as a raw material of a silicon thin film, a mixing of oxygen is caused and this adversely affects the characteristics of a semiconductor or fiber. Accordingly, high-purity SiF4 reduced in impurities is required and, as one of the evaluation techniques, an analysis method for trace impurities is also required.
With respect to the method for purifying SiF4, for example, Japanese Unexamined Patent Publication No. 57-156317 (JP-A-57-156317) describes a method for purifying SiF4 containing (SiF3)2O by contacting it with an adsorbent. However, when the adsorbent used in this method is regenerated and used, the initial adsorbing capability is not brought out in some cases. The reason therefor is not clearly known but it is considered to be because adsorbed hexafluorodisiloxane is decomposed within pores of the adsorbent. As SiO2 produced by the decomposition attaches to the adsorption site, the adsorbent cannot be regenerated and re-used and this causes a problem that the adsorbent must be treated as a waste. Furthermore, if the adsorbent is not sufficiently baked before the passing of gas, a side reaction with water content takes place to produce hexafluorodisiloxane instead.